A microwave oven is a kitchen appliance that employs microwave radiation primarily to cook or heat food. The microwave oven is designed such that microwave radiation is oscillated from a magnetron and radiated into a cooking chamber by being guided by a waveguide. The cooking chamber is provided with a radiation opening through which the microwave radiation guided by the waveguide is radiated into the cooking chamber. However, a size of the radiation opening is a major factor that determines the radiation uniformity of the microwave. However, the related art is not reflecting this consideration.